Europium can exist in two oxidation states when present in glass. The more common trivalent state, Eu.sup.+3, emits a moderately strong reddish photoluminescence, i.e., upon exposure to ultraviolet radiation, particularly where europium is included in high concentrations; but its cathodoluminescence, i.e., its emission when subjected to cathode rays, is quite weak. The divalent state, Eu.sup.+2, can emit a strong bluish luminescence under both forms of excitation, viz., ultraviolet and cathode rays. However, in many conventionally melted, high temperature glasses a mixture of the two valence states will exist such that the luminescence color exhibited under ultraviolet or cathode ray excitation is the result of contributions from both valence states. Hence, luminescence colors can range from violet or violet-blue at low europium concentrations, thereby indicating a predominance of Eu.sup.+2 emission, to deep pink or red at high europium concentrations, thereby evidencing a predominance of Eu.sup.+3 emission. However, the cathodoluminescent efficiency of such glasses is, in the main, very poor. It has been postulated that the presence of the relatively inefficient Eu.sup.+3 ions degrades the efficiency of the Eu.sup.+2 ions which are present in the glass. Enhancement of the Eu.sup.+2 ion concentration in the glass has been attempted via melting the glass batch under reducing conditions, but such trials have not been uniformly satisfactory and, furthermore, the procedures required are cumbersome and not readily adaptable to commercial glass melting operations.
It has long been recognized that crystals commonly luminesce with much higher efficiencies than do glasses of like composition. Nevertheless, the transparency and freedom from light-scattering defects intrinsically present in glass articles have stimulated continued research to develop glass compositions which will luminesce brilliantly when subjected to a beam of high voltage electrons. Such glasses would be particularly suitable for the formation of transparent faceplates or screens for cathode ray tubes or other applications where image contrast or image resolution is of vital importance.
Unfortunately, with few exceptions, this research has not been rewarding in that the glasses developed have generally demonstrated poor cathodoluminescence efficiency. For example, the large majority of the glasses produced exhibited efficiencies of less than about 1%. Therefore, a cathodoluminescence efficiency of 1% has been deemed a baseline for certifying an improved glass.